This invention relates to a heat developable silver image forming material. More particularly, it relates to a heat developable silver image forming material having good sensitivity and storage stability capable of forming a silver image thereon by reduction of the silver ion of an organic silver salt oxidizing agent with a reducing agent for silver ion by heat development.
2. Description of the Prior Art
Conventional wet processing photosensitive materials containing a silver halide require complicated and time consuming processes using various treating chemicals in obtaining stable images and further, the chemicals used give rise to a pollution problem. Accordingly, in order to avoid these disadvantages there have been made many attempts to develop photosensitive materials having high sensitivity capable of forming stable images thereon only by dry processing without using treating chemicals. For example, U.S. Pat. No. 3,152,902 describes a heat developable silver image forming material comprising an organic silver salt having rather good light stability, a reducing agent for silver ion and a photosensitive silver halide. U.S. Pat. No. 3,457,075 describes a heat developable silver image photosensitive material whose sensitivity is increased by the photosensitive silver halide formed by halogenation of a small portion of the organic silver salt. However, the sensitivity of these photosensitive materials is still insufficient for practical purposes and the storage stability thereof are markedly low. Further, U.S. Pat. Nos. 3,802,888 and 3,764,329 describe heat developable silver image forming materials which are non-photosensitive under normal light conditions and can be rendered photosensitive by the preliminary heating prior to the imagewise exposure to light. The heat developable silver image forming materials of this type are hereinafter referred to as "post activatable type heat developable silver image forming materials". These post activatable type heat developable silver image forming materials can be advantageously prepared and handled in normal light conditions, but according to U.S. Pat. No. 3,802,888, their minimum optical density (fog density) increase when they are stored under normal light conditions, and hence their storage stability (shelf life) is still insufficient for practical purposes. The post activable type heat developable silver image forming materials described in U.S. Pat. No. 3,764,329 have higher storage stability but instead lower sensitivity than those of U.S. Pat. No. 3,802,888. In general, use of a sensitizer reduces the storage stability, increases the minimum optical density and decreases or, in some cases, increases the sensitivity. The change of the sensitivity due to their storage under normal light conditions is also undesirable for post activable type heat developable silver image forming materials.
Furthermore, the heat developable silver image forming materials produce images by reduction of the silver ion of an organic silver salt upon the imagewise exposure to light, followed by heat development and differs from conventional silver halide photosensitive materials in their image forming mechanism as well as in their compositions. Thus, it is well known in this field that the techniques for increasing the sensitivity or the storage stability of conventional silver halide photosensitive materials cannot be applied directly for heat developable silver image forming materials.